The invention relates to a multi-beam scanning device that simultaneously scans a plurality of light beam on an object.
Optical image forming devices such as laser printers and digital copy machines form images on a photoconductive drum by scanning a light beam thereon. Recently, such optical image forming devices are provided with a multi-beam scanning device in which a plurality of light beams are emitted toward a single polygon mirror and deflected toward the photoconductive drum so that those plurality of light beams are simultaneously scanned across the photoconductive drum. By simultaneously scanning a plurality of light beams as above, the optical image forming devices are enhancing the image forming rate.
Japanese Patent provisional publication HEI 8-304722 discloses a multi-beam scanning device that simultaneously scans a plurality of light beams with a single polygon mirror. In the disclosed device, two light beams hit the polygon mirror after being arranged parallel and adjacent to each other by utilizing a beam splitter. This multi-beam scanning device, however, raise the cost of the optical image forming device since the beam splitter, which is a relatively expensive optical element, is utilized.
Japanese Patent provisional publication P2000-249948 discloses a multi-beam scanning device in which two light beams are incident on the same location of the polygon mirror at different incident angles. This device can be produced at lower cost than the one disclosed in Japanese Patent provisional publication HEI 8-304722 since it does not utilizes any beam splitters to place the light beam parallel and adjacent to each other.
However, since the light beams are incident on the polygon mirror at different incident angles, the range of the polygon mirror rotating angle that is required to scan the light beam across the photoconductive drum differs between the two light beams. The difference in the ranges of the polygon mirror rotating angle results in different characteristics of the bowing of the scan line and/or the scanning speed of the two light beams and causes deterioration of the quality of the image formed. Further, the angle range difference also requires the use of an large polygon mirror, which raise the cost of the production, since the reflecting surface of the polygon mirror have to be wide enough to achieve the scanning of both of the two light beams. Furthermore, the use of the large polygon mirror requires a high power motor for rotating the polygon mirror in high revolving speed, which also raise the cost of the production.
To avoid the disadvantages mentioned above, it is required to minimize the difference of the incident angles of the light beams impinging on the polygon mirror. However, in the device disclosed in the Japanese Patent provisional publication P2000-249948, the difference between the incident angles cannot be made smaller than a value that is determined by the diameters of collimators located on the optical paths of the two light beam.
The present invention is advantageous in that a multi-beam scanning device is provided which has a simple optical configuration and can be produced in low cost.
According to an aspect of the invention, there is provided a multi-beam scanning device that includes first and second light emitting elements that emit first and second light beams, respectively, a polygon mirror that deflects the first and second light beams to simultaneously scan the first and second light beams across an object, and a first prism having a light entrance portion and a light exit portion. The first light beam enters the first prism through the light entrance portion and exits from the first prism through the light exit portion toward the polygon mirror. The light exit portion is inserted into the optical path of the second light beam to prevent a part of the second light beam from proceeding toward the polygon mirror.
In the multi-beam scanning device configured as above, the first light beam exiting from the first prism is located close to the second light beam with substantially no gap therebetween. As a result, the first and second light beams impinge on the polygon mirror at incident angles of which difference is quite small and therefore the first and second light beams can be scanned across the object with a small and cheap polygon mirror.
The first prism may be configured such that the light exit portion includes a reflection plane provided with a reflection layer so that it reflects the first light beam toward the polygon mirror while blocking the part of the second light beam. Alternatively, the first prism may be configured such that the light exit portion includes a reflection plane which reflects the first light beam impinging thereon toward the deflector by total internal reflection and refracts the part of the second light beam so that it does not proceed toward the polygon mirror.
In some cases, the light exit portion includes a corner of the first prism which has a chamfered edge. One of the plane that is defining the corner is a reflection plane that reflects the first light beam toward the polygon mirror. The first light beam impinges on the reflection plane such that a part of the first light beam impinges on the chamfered edge. The surface of the chamfered edge may be finished such that the first light beam incident thereon is scattered. Further, the first prism may include a blocking groove that restricts the amount of light of the first light beam incident on the chamfered edge.
In some cases, the multi-beam scanning device includes a third light emitting element that emits a third light beam, and a second prism having a light entrance portion and a light exit portion. The third light beam enters the second prism through the light entrance portion and exits from the third prism through the light exit portion toward the polygon mirror. The light exit portion is inserted into the optical path of the second light beam to prevent a part of the second light beam from proceeding toward the polygon mirror. The first and second prisms are arranged to define a gap between the exit portions thereof. The second light beam passes through the gap. The width of the second light beam proceeding toward the polygon mirror is restricted by the gap.
In the above case, the multi-beam scanning device may further include a slit located on the optical paths of the first and third light beams extending from the first and second prisms to the polygonal mirror. The slit adjusting the width of the first and third light beams to be the same as the width of the second light beam.
In some cases, the multi-beam scanning device has a single supporting member, which may be made from a material having high heat conductivity, and the first, second and third light emitting elements are supported by this single supporting member. The first and second prisms may be also supported by the same supporting member.
According to another aspect of the invention, a light source is provided that includes first and second light emitting elements that emit first and second light beams, respectively, a first prism, and a single supporting member that supports the first and second light emitting elements and the first prism. The first prism is located such that the first light beam enters the first prism. The first prism has a light exit portion through which the first light beam exits from the first prism in a predetermined direction. The light exit portion is inserted into the optical path of the second light beam to prevent a part of the second light beam from proceeding in the predetermined direction.